Parking-Lot Sealcoat: A Major Source of PAHs in Urban and Suburban Environments

Coal-tar based sealcoat—the black, shiny emulsion painted or sprayed on asphalt pavement such as parking lots—has extremely elevated concentrations of polycyclic aromatic hydrocarbons (PAHs) and can affect the quality of downstream water resources, according to a recent joint study in Texas by the USGS National Water-Quality Assessment (NAWQA) Program and the City of Austin. PAHs are an environmental concern because they are toxic to aquatic life and because several are suspected human carcinogens. Small particles of sealcoat flake off as they are abraded by vehicle tires, and can wash into urban streams with rain and runoff. The study found that particles in runoff from coal-tar based sealcoated parking lots have PAH concentrations that are about 65 times higher than in particles washed off parking lots that have not been sealcoated. Particles in runoff from parking lots sealed with asphalt-based sealcoat, the other major product on the market, have PAH concentrations about 10 times higher than those from unsealed lots. The large differences suggest that abraded sealcoat is a potentially dominant (and heretofore unrecognized) source of PAHs in urban and suburban water bodies. PAH concentrations have been increasing over the past 30-35 years in many urban and suburban lakes across the United States.